Effects of the combined application of organic and chemical fertilizers on soil microbial biomass and respiration in saline soils

Microorganisms adapt to saline environments by utilizing energy to produce organic osmolytes. However, their energy requirements may fluctuate with varying soil salinity levels. The study focused on maize cultivation in soils with slight and moderate salinity (S1 and S2; Electrical Conductivity [EC] of 0.46 and 1.07 dS∙m^-1 , respectively). Six treatments were applied: CK (no fertilization); U1 (urea); and U₃O₁, U₁O₁, U₁O₃, and O1, where different proportions of urea nitrogen were substituted with organic fertilizers. As soil salinity increased, soil microbial biomass and microbial activity decreased. MBC, MBN, and soil respiration flux were 12.01%-68.81%, 14.31%-58.58%, and 11.75%-54.71% lower in the S₂ soil than in S₁ soil. In both soils, soil microbial biomass and microbial activity were significantly increased when the appropriate proportion of organic fertilizer was applied. MBC, MBN, in the S₁ soil were higher under U₁O₁ treatment than under U₁ treatment—by 48.44%, 42.50%, and 31.74%, respectively (P ⪅ 0.05). In the S2 soil, MBC, MBN, and soil respiration flux were significantly higher under O₁ treatment than under U₁ treatment—by 68.07%, 48.99%, and 45.19%, respectively (P ⪅ 0.05). Correlation analysis revealed significant positive correlations between soil respiration and soil microbial biomass (P ⪅ 0.01). Soil temperature and nitrogen content were positively correlated with soil respiration and soil microbial biomass (P ⪅ 0.05).